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The high Landau levels have demonstrated numerous interesting phenomena with theoretical models in need of specific experimental support. While the composite fermion picture provides a model for the series of correlated electron liquids states manifesting as the fractional quantum Hall effect in the lowest Landau level, this model finds its limits in the higher Landau levels. Here transport phenomena such as anisotropic conductance at the half-filled levels dominate the magnetoresistance spectra, which is ostensibly due to phase separation into stripes of integral filling. In the B-field range between these anisotropy findings in the high Landau levels and the FQHE of the lowest Landau level, the anomalous state at 5 2 filling factor has presented an enigmatic variation on the standard liquid states normally found exclusively at odd-denominator filling. This unique state has been suggested to result from composite fermion pairing in d or p wave. In this work we show results independently addressing these theoretical models for the striped phase and the 5 2 state. We show magnetoresistance data that specifically indicates striped phase formation in the 2DES, also depicting the direction of the stripes within the sample. At smaller filling factors in surface acoustic wave measurements we show evidence for the composite fermion model of pairing to form the 5 2 state by demonstrating Fermi surface formation at temperatures above those at which the 5 2 is quantized in Hall resistance.

We dynamically modulate strong light–matter coupling in a GaAs/AlGaAs microcavity using intense ultrashort laser pulses tuned below the interband exciton energy, which induce a transient Stark shift of the cavity polaritons. For 225-fs pulses, shorter than the cavity Rabi cycle period of 1000 fs, this shift decouples excitons and cavity photons for the duration of the pulse, interrupting the periodic energy exchange between photonic and electronic states. For 1500-fs pulses, longer than the Rabi cycle period, however, the Stark shift does not affect the strong coupling. The two regimes are marked by distinctly different line shapes in ultrafast reflectivity measurements—regardless of the Stark field intensity. The crossover marks the transition from adiabatic to diabatic switching of strong light–matter coupling.

In the region of the second Landau level several theories predict fractional quantum Hall states with novel topological order. We report the opening of an energy gap at the filling factor $\ensuremath{\nu}=3+1/3$, firmly establishing the ground state as a fractional quantum Hall state. This and other odd-denominator states unexpectedly break particle-hole symmetry. Specifically, we find that the relative magnitudes of the energy gaps of the $\ensuremath{\nu}=3+1/3$ and $3+1/5$ states from the upper spin branch are reversed when compared to the $\ensuremath{\nu}=2+1/3$ and $2+1/5$ counterpart states in the lower spin branch. Our findings raise the possibility that at least one of the former states is of an unusual topological order.

In a quasi-two-dimensional electron system with nonzero layer thickness, a parallel magnetic field can couple to the out-of-plane electron motion and lead to a severe distortion and eventual splitting of the Fermi contour. Here we directly and quantitatively probe this evolution through commensurability and Shubnikov-de Haas measurements on electrons confined to a 40-nm-wide GaAs (001) quantum well. We are able to observe the Fermi contour splitting phenomenon, in good agreement with the results of semiclassical calculations. Experimentally, we also observe intriguing features, suggesting magnetic-breakdown-type behavior when the Fermi contour splits.

Clean two-dimensional electron systems in GaAs/AlGaAs heterostructures exhibit anisotropic collective phases, the quantum Hall nematics, at high Landau level occupancy and low temperatures. An as yet unknown native symmetry-breaking potential consistently orients these phases relative to the crystalline axes of the host material. Here we report an extensive set of measurements examining the role of the structural symmetries of the heterostructure in determining the orientation of the nematics. In single quantum well samples we find that neither the local symmetry of the confinement potential nor the distance between the electron system and the sample surface dictates the orientation of the nematic. In remarkable contrast, for two-dimensional electrons confined at a single heterointerface between GaAs and AlGaAs, the nematic orientation depends on the depth of the two-dimensional electron system beneath the sample surface., Comment: 9 pages, 5 figures. Minor revision to acknowledgements

We modulate strong coupling in a GaAs/AlGaAs microcavity by transiently decoupling exciton and cavity through the optical Stark effect induced by intense, red-detuned laser pulses. Pulses longer (1500 fs) and shorter (225 fs) than the Rabi cycle time of 500 fs yield distinctly different line shapes.

We present in this Letter the results from two high quality, low density GaAs quantum wells. In sample A of electron density n=5.010 10 cm -2 , anisotropic electronic transport behavior was observed at =7/2 in the second Landau level. We believe that the anisotropy is due to the large Landau level mixing effect in this sample. In sample B of density 4.110 10 cm -2 , strong 8/3, 5/2, and 7/3 fractional quantum Hall states were observed. Furthermore, our energy gap data suggest that, similar to the 8/3 state, the 5/2 state may also be spin unpolarized in the low density limit. The results from both samples show that the strong electron-electron interactions and a large Landau level mixing effect play an import role in the competing ground states in the second landau level.

By combining a tilted-pulse-intensity-front scheme using a LiNbO3 crystal and a chirped-pulse-beating method, we generated a narrowband intense terahertz (THz) pulse, which had a maximum electric field of more than 10 kV/cm at around 2 THz, a bandwidth of about 50 GHz,and frequency tunability from 0.5 to 2 THz. By performing THz-pump and near-infrared-probe experiments on GaAs quantum wells, we observed that the resonant excitation of the intraexcitonic 1s-2p transition induces a clear and large Autler-Townes splitting. Our time-resolved measurements show that the splitting energy observed in the rising edge region of electric field is larger than in the constant region. This result implies that the splitting energy depends on the time-averaged THz field over the excitonic dephasing time rather than that at the instant of the exciton creation by a probe pulse.

We realize a nanoscale-area Mach-Zehnder interferometer with co-propagating quantum Hall spin-resolved edge states and demonstrate the persistence of gate-controlled quantum interference oscillations, as a function of an applied magnetic field, at relatively large temperatures. Arrays of top-gate magnetic nanofingers are used to induce a resonant charge transfer between the pair of spin-resolved edge states. To account for the pattern of oscillations measured as a function of magnetic field and gate voltage, we have developed a simple theoretical model which satisfactorily reproduces the data., Comment: 7 pages, 5 figures

A near-field probe is described that enables high spatial resolution imaging with terahertz (THz) pulses. The spatial resolution capabilities of the system lie in the range of few microns and we demonstrate a resolution of 7 microm using broad-banded THz pulses with an intensity maximum near 0.5 THz. We present a study of the performance of the near-field probes in the collection mode configuration and discuss some image properties.

During the past ten years, coupled quantum wells have emerged as a promising system for experiments on Bose condensation of excitons, with numerous theoretical1,2,3,4,5,6 and experimental7,8,9,10,11,12 studies aimed at the demonstration of this effect. One of the issues driving these studies is the possibility of long-range coherent transport of excitons. Excitons in quantum wells typically diffuse only a few micrometres from the spot where they are generated by a laser pulse; their diffusion is limited by their lifetime (typically a few nanoseconds) and by scattering due to disorder in the well structure. Here we report photoluminescence measurements of InGaAs quantum wells and the observation of an effect by which luminescence from excitons appears hundreds of micrometres away from the laser excitation spot. This luminescence appears as a ring around the laser spot; almost none appears in the region between the laser spot and the ring. This implies that the excitons must travel in a dark state until they reach some critical distance, at which they collectively revert to luminescing states. It is unclear whether this effect is related to macroscopic coherence caused by Bose condensation of excitons.

The recently developed AFM local anodic oxidation (LAO) technique offers a convenient way of patterning nanodevices, but imposes even more stringent requirements on the underlying quantum well structure. We developed a new very shallow quantum well design which allows the depth and density of the 2D gas to be independently controlled during the growth. A high mobility ( 0.5×10 6 cm 2 V −1 s −1 at 4.2 K) 2D hole gas just 350 A below the surface is demonstrated. A quantum point contact, fabricated by AFM LAO nanopatterning from this wafer, shows nine quantum steps at 50 mK.

We report on an extensive study of the growth and transport properties of the two-dimensional electron gas (2DEG) confined at the interface of AlGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) on thick, semi-insulating GaN templates prepared by hydride vapor phase epitaxy (HVPE). Thick (∼20μm) GaN templates are characterized by low threading dislocation densities (∼5×10 ⁸cm ⁻²) and by room temperature resistivities of ∼10 ⁸cm. We describe sources of parasitic conduction in our structures and how they have been minimized. The growth of low Al containing (x≤0.05) Al ₓGa ₁₋ₓN/GaN heterostructures is investigated. The use of low Al containing heterostructures facilitates the study of the 2DEG transport properties in the previously unexplored regime of carrier density n s≤2×10 ¹²cm ⁻². We detail the impact of MBE growth conditions on low temperature mobility. Using an undoped HVPE template that was residually n type at room temperature and characterized an unusually low dislocation density of ∼2×10 ⁸cm ⁻², we have grown an Al ₀.₀₅Ga ₀.₉₅N/GaN heterostructure with a record mobility of 75000cm ²/Vs at sheet density of 1.5×10 ¹²cm ⁻² and T=4.2K. The same heterostructure design grown on a semi-insulating HVPE template yielded a peak mobility of 62000cm ²/Vs at a density of n s=1.7×10 ¹²cm ⁻² and T=4.2K. The observation of the fractional quantum Hall effect at filling factor ν=5/3 in the AlGaN/GaN system is reported. It is also demonstrated that thick semi-insulating GaN templates grown by HVPE are a viable substrate for the growth of high electron mobility transistors. Typical Al ₀.₂₅Ga ₀.₇₅N/GaN heterostructures exhibit room temperature density of 1.0×10 ¹³cm ⁻³ and mobility of ∼1500cm ²/Vs. The dc and rf characteristics of transistors grown by MBE on a HVPE template are presented. © 2002 American Institute of Physics.

We demonstrate strong one dimensional (1-D) many-body interaction effects in photoluminescence (PL) in a GaAs single quantum wire of unprecedented optical quality, where 1-D electron plasma densities are controlled via electrical gating. We observed PL of 1-D charged excitons with large binding energy of 2.3 meV relative to the neutral excitons, and its evolution to a Fermi-edge singularity at high electron density. Furthermore, we find a strong band-gap renormalization in the 1-D wire, or a large red-shift of PL with increased electron plasma density. Such a large PL red-shift is not observed when we create a high density neutral electron-hole plasma in the same wire, due probably to cancellation of the Coulomb interaction energy in the neutral plasma., 5 pages, 4 figures, RevTeX, to be published in Solid State Communications

Intersubband electroluminescence is reported in a quantum-cascade structure based on asymmetric superlattice active regions and designed for emission in the THz range (λ≈80 μm). Comparison with a structure based on a “vertical transition” in a single quantum well shows an increased full width at half maximum (2.8 vs 0.9 meV) of the emission line. In both cases the dependence of the optical power on the injected current is linear or sublinear and remains in the pW range.

This paper presents a short review of localisation in strongly interacting, high quality dilute 2D GaAs systems. At zero magnetic field, studies of the temperature dependent resistance of both 2D electron and hole systems show a transition from insulating to metallic behaviour with increasing carrier density. However, careful examination of the 2D hole systems reveal the presence of localising quantum corrections to the conductivity which persist down to the lowest measurement temperatures. Our results highlight the importance of avoiding electron/hole heating at low temperatures and argue against the existence of a 2D metallic phase at B = 0.

Quantum cascade lasers operating at λ>20 μm wavelength are reported. Pulsed operation was obtained up to 140 K with a peak power of few milliwatts at cryogenic temperatures. Laser action originates from interminiband transitions in “chirped” superlattice active regions. The waveguides are based on surface-plasmon modes confined at a metal–semiconductor interface, but we also report on a 21.5 μm wavelength laser based on a double-sided interface-plasmon waveguide. This latter, contrary to the single-sided surface plasmon, is a viable waveguiding solution in the THz range, i.e. at wavelengths between 60 and 100 μm . Finally, intersubband electroluminescence is reported in a quantum cascade structure based on asymmetric superlattice active regions and designed for emission in the THz range at λ≈80 μm .

We report the carrier dynamics of 1.3-μm InAs quantum dots (QDs), following resonant excitation in the lowest energy state of QDs. The strong temperature dependence of the escape rates of the carriers leaving the ground state shows the presence of efficient multiphonon processes, involving both acoustic and optical phonons. The very fast activation of electrons to the first excited state implies that, in these QDs, the phonon bottleneck is not observable at room temperature even at low carrier densities where the dynamics is independent of excitation level.

New tunnel current measurements of resonant tunneling at a voltage-biased quantum Hall effect edge are reported and explained with an in-depth discussion of the lever-arm model proposed previously by the authors. The results support the power-law description of the density of states at a sharp fractional quantum Hall edge.

We have investigated the photoluminescence (PL) and inelastic light-scattering spectra of dilute two-dimensional (2D) hole gases of ultra-high mobility. The samples were GaAs–Al x Ga 1− x As (3 1 1) A quantum structures, modulation-doped with Si. We find these samples not to deplete fully on illumination. The PL displays both excitonic and 2D hole gas transitions. We observe inelastic light-scattering, resonating on these intersubband transitions. Our measured intersubband excitation energies are in agreement with current calculations. These resonance enhancements are consistent with an electron-like dispersion of the first-excited hole subband at zero magnetic field.

Cleaved-edge overgrown semiconductor quantum wires containing a one-dimensional electron gas with few occupied subbands are probed by optical experiments. Inter-sub-band excitation modes are observed by resonant inelastic light scattering at energies that agree well with theoretical estimates obtained within the Fermi-liquid model. Narrow spectral line-shapes indicate high uniformity and long electron scattering lengths that are consistent with recent transport measurements.

Magnetorotons in the dispersion curves of collective gap excitation modes of fractional quantum Hall liquids are measured by resonant inelastic light scattering. Two deep magnetoroton minima are observed at ν=25, while a single deep minimum is resolved at ν=13. The results support Chern–Simons and composite fermion calculations that predict multiple roton minima for states with ν>13.

We compare micro-photoluminescence (μPL) as a measure of the electron density in a clean, two-dimensional (2D) system confined in a GaAs quantum well (QW) to the standard magneto-transport technique. Our study explores the PL shape evolution across a number of molecular beam epitaxy-grown samples with different QW widths and 2D electron densities and notes its correspondence with the density obtained in magneto-transport measurements on these samples. We also measure the 2D density in a top-gated quantum well sample using both PL and transport and find that the two techniques agree to within a few percent over a wide range of gate voltages. We find that the PL measurements are sensitive to gate-induced 2D density changes on the order of 109 electrons/cm2. The spatial resolution of the PL density measurement in our experiments is 40 μm, which is already substantially better than the millimeter-scale resolution now possible in spatial density mapping using magneto-transport. Our results establish that μPL c...

We present well-developed low-field magneto-resistance oscillations originating from zero-field spin splitting (ZFSS) of heavy holes in high mobility GaAs/AlGaAs quantum wells. This low field oscillation is 1/B-periodic and emerges before the onset of Shubnikov–de Haas oscillations. The effect can be explained by resonant scattering between two Landau ladders shifted by the ZFSS gap, which in turn can be measured by comparing with the hole cyclotron energy. A front gate is fabricated to tune the ZFSS and hence the oscillation period.

Magnetotransport experiments including tilt fields are performed on ultrahigh mobility L-shaped Hall-bar samples of GaAs/AlGaAs quantum wells. The low-temperature longitudinal resistivity (ρxx ) data demonstrate that a striking even–odd asymmetric transport exists along the [ ] direction at half filling in N ≥ 2 high Landau levels. Although the origin for the peculiar even–odd asymmetry remains unclear, we propose that the coupling strength between electrons within the same Landau level and between the neighboring two Landau levels should be considered in future studies. The tilt field data show that the in-plane field can suppress the formation of both bubble and stripe phases.

In two-dimensional electron systems confined to GaAs quantum wells, as a function of either tilting the sample in a magnetic field or increasing density, we observe multiple spin-polarization transitions of the fractional quantum Hall states at filling factors ν=4/5 and 5/7. The number of observed transitions provides evidence that these are fractional quantum Hall states of interacting two-flux composite fermions. Moreover, the fact that the reentrant integer quantum Hall effect near ν=4/5 always develops following the transition to full spin polarization of the ν=4/5 fractional quantum Hall state links the reentrant phase to a pinned ferromagnetic Wigner crystal of composite fermions.

Search for Majorana fermions renewed interest in semiconductor–superconductor interfaces, while a quest for higher-order non-Abelian excitations demands formation of superconducting contacts to materials with fractionalized excitations, such as a two-dimensional electron gas in a fractional quantum Hall regime. Here we report induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures and development of highly transparent semiconductor–superconductor ohmic contacts. Supercurrent with characteristic temperature dependence of a ballistic junction has been observed across 0.6 μm, a regime previously achieved only in point contacts but essential to the formation of well separated non-Abelian states. High critical fields (>16 T) in NbN contacts enables investigation of an interplay between superconductivity and strongly correlated states in a two-dimensional electron gas at high magnetic fields., Superconductivity has been induced in 2D electron gases, but high-field interplay between it and quantum Hall edge states remains elusive. Here the authors reach this regime by growing transparent superconducting contacts in GaAs, reporting modification of resistance in the quantum Hall regime.

We report ballistic transport commensurability minima in the magnetoresistance of $\nu =3/2$ composite fermions (CFs). The CFs are formed in high-quality two-dimensional electron systems confined to wide GaAs quantum wells and subjected to an in-plane, unidirectional periodic potential modulation. We observe a slight asymmetry of the CF commensurability positions with respect to $\nu=3/2$, which we explain quantitatively by comparing three CF density models and concluding that the $\nu=3/2$ CFs are likely formed by the minority carriers in the upper energy spin state of the lowest Landau level. Our data also allow us to probe the shape and size of the CF Fermi contour. At a fixed electron density of $\simeq 1.8 \times 10^{11}$ cm$^{-2}$, as the quantum well width increases from 30 to 60 nm, the CFs show increasing spin-polarization. We attribute this to the enhancement of the Zeeman energy relative to the Coulomb energy in wider wells where the latter is softened because of the larger electron layer thickness. The application of an additional parallel magnetic field ($B_{||}$) leads to a significant distortion of the CF Fermi contour as $B_{||}$ couples to the CFs' out-of-plane orbital motion. The distortion is much more severe compared to the $\nu=1/2$ CF case at comparable $B_{||}$. Moreover, the applied $B_{||}$ further spin-polarizes the $\nu=3/2$ CFs as deduced from the positions of the commensurability minima.

We report on nonlinear transport measurements in a GaAs/AlGaAs quantum well exhibiting a colossal negative magnetoresistance effect. Under applied dc bias, the magnetoresistance becomes nonmonotonic, exhibiting distinct extrema that move to higher magnetic fields with increasing current. In the range of magnetic fields corresponding to the resistivity minimum at zero bias, the resistivity increases linearly with current and the rate of this increase scales with the inverse magnetic field. The latter observation is consistent with the theory, proposed more than 35 years ago, considering classical memory effects in the presence of strong, dilute scatterers.

Quantum Hall measurements are performed in a dilute high-quality GaAs two-dimensional hole system with carrier densities of (4.4--$5.3)\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.28em}{0ex}}{\text{cm}}^{\ensuremath{-}2}$. With the ${r}_{s}$ value approximately 20, the interaction effect is already strong even in the absence of magnetic field. In the integer quantum Hall regime up to 2 T, a series of reentrant insulating phases (RIPs) are observed between fillings 1, 2, 3, and 4, with peak locations periodic in $1/B$. Moreover, the RIP peaks are also characterized by a striking inductance in response to ac excitations, which is likely due to the onset of a Wigner crystal state. The multiple RIP peaks indicate a more complex phase diagram than previously perceived.

Vertical heterostructures combining different layered materials offer novel opportunities for applications and fundamental studies. Here we report a new class of heterostructures comprising a single-layer (or bilayer) graphene in close proximity to a quantum well created in GaAs and supporting a high-mobility two-dimensional electron gas. In our devices, graphene is naturally hole-doped, thereby allowing for the investigation of electron-hole interactions. We focus on the Coulomb drag transport measurements, which are sensitive to many-body effects, and find that the Coulomb drag resistivity significantly increases for temperatures

We resonantly inject polaritons into a microcavity and track them in time and space as they feel a force due to the cavity gradient. This is an example of "slow reflection," as the polaritons, which can be viewed as renormalized photons, slow down to zero velocity and then move back in the opposite direction. These measurements accurately measure the lifetime of the polaritons in our samples, which is 180 $\pm$ 10 ps, corresponding to a cavity leakage time of 135 ps and a cavity $Q$ of 320,000. Such long-lived polaritons propagate millimeters in these wedge-shaped microcavities. Additionally, we generate polaritons by two-photon excitation directly into the polariton states, allowing the possibility of modulation of the two-photon absorption by a polariton condensate., 4 figures. Supplemental info included after bibliography

In high magnetic fields, two-dimensional electron systems can form a number of phases in which interelectron repulsion plays the central role, since the kinetic energy is frozen out by Landau quantization. These phases include the well-known liquids of the fractional quantum Hall effect, as well as solid phases with broken spatial symmetry and crystalline order. Solids can occur at the low Landau-filling termination of the fractional quantum Hall effect series but also within integer quantum Hall effects. Here we present microwave spectroscopy studies of wide quantum wells that clearly reveal two distinct solid phases, hidden within what in d.c. transport would be the zero diagonal conductivity of an integer quantum-Hall-effect state. Explanation of these solids is not possible with the simple picture of a Wigner solid of ordinary (quasi) electrons or holes., Confining electrons to two dimensions can drastically alter the way they interact with each other. Here, the authors show that electrons in quantum wells can form into two distinct solid-like phases when placed in a large magnetic field.

In this Rapid Communication we present the results from two high-quality, low-density GaAs quantum wells. In sample A of electron density $n$ = 5.0 \ifmmode\times\else\texttimes\fi{} 10${}^{10}$ cm${}^{\ensuremath{-}2}$, anisotropic electronic transport behavior was observed at \ensuremath{\nu} = 7/2 in the second Landau level. We believe that the anisotropy is due to the large Landau level mixing effect in this sample. In sample B of density 4.1 \ifmmode\times\else\texttimes\fi{} 10${}^{10}$ cm${}^{\ensuremath{-}2}$, strong 8/3, 5/2, and 7/3 fractional quantum Hall states were observed. Furthermore, our energy gap data suggest that, similar to the 8/3 state, the 5/2 state may also be spin unpolarized in the low-density limit. The results from both samples show that the strong electron-electron interactions and a large Landau level mixing effect play an import role in the competing ground states in the second Landau level.

Composite fermions (CFs), exotic particles formed by pairing an even number of flux quanta to each electron, provide a fascinating description of phenomena exhibited by interacting two-dimensional electrons at high magnetic fields. At and near Landau level filling $\nu=1/2$, CFs occupy a Fermi sea and exhibit commensurability effects when subjected to a periodic potential modulation. We observe a pronounced asymmetry in the magnetic field positions of the commensurability resistance minima of CFs with respect to the field at $\nu=1/2$. This unexpected asymmetry is quantitatively consistent with the CFs' Fermi wave vector being determined by the \textit{minority} carriers in the lowest Landau level. Our data indicate a breaking of the particle-hole symmetry for CFs near $\nu=1/2$., Comment: 4 pages, 4 figures

An imaging and loss evaluation method for semiconductor waveguides coupled with non-doped quantum wells is presented. Using the internal emission of the wells as a probe light source, the numbers and widths of the modes of waveguides with various ridge sizes were evaluated by CCD imaging, and the obtained values were consistent with effective index method calculation. Waveguide internal losses were obtained from analyses of the Fabry-Pérot fringes of waveguide emission spectra. We quantified the quality of 29 single-mode waveguide samples as an internal loss and variation of 10.2 ± 0.6 cm(-1).

We report on a colossal negative magnetoresistance (MR) in a GaAs/AlGaAs quantum well which, at low temperatures, is manifested by a drop of the resistivity by more than an order of magnitude at a magnetic field $B\ensuremath{\approx}1$ kG. In contrast to MR effects discussed earlier, the MR reported here is not parabolic, even at small $B$, and persists to much higher in-plane magnetic fields and temperatures. Remarkably, the temperature dependence of the resistivity at $B\ensuremath{\approx}1$ kG is linear over the entire temperature range studied (from 1 to 30 K) and appears to coincide with the high-temperature limit of the zero-field resistivity, hinting on the important role of acoustic phonons.

We report that excitonic lasing gain coexists with spontaneous optical emission characteristic of an electron–hole plasma in highly photoexcited one-dimensional semiconductors. The experiments probe quantum T-wire laser structures optimized for high photoexcitation. Evidence of dense electron–hole plasma is clearly seen in the spontaneous recombination measured when lasing emission displays distinct excitonic character. These findings differ strikingly from those in higher dimentional semiconductors, and offer insights on optical processes considered by recent theories of dense electron–hole plasmas.

In the quest to understand (and to make use of) spin effects in mesoscopic systems, the development of new experimental techniques will play a key role. In this review, we will describe some recent work using Optically Pumped Nuclear Magnetic Resonance (OPNMR), a novel spectroscopy that has proven to be an useful probe of electron spin physics in the quantum Hall regimes. We will show: (1) the OPNMR signal from a single GaAs quantum well, (2) the ‘motionally narrowed’ lineshape reflects the variation of the electron density across the ∼300 A well, and (3) the dramatic changes in the lineshape which provide evidence for Skyrmion localization along the plane of the well.

Optically pumped nuclear magnetic resonance measurements of 71Ga spectra were carried out in an n-doped GaAs/Al(0.1)Ga0.9As multiple quantum well sample near the integer quantum Hall ground state nu = 1. As the temperature is lowered (down to T approximately 0.3 K), a "tilted plateau" emerges in the Knight shift data, which is a novel experimental signature of quasiparticle localization. The dependence of the spectra on both T and nu suggests that the localization is a collective process. The frozen limit spectra appear to rule out a 2D lattice of conventional Skyrmions.

This paper presents some recent results on the anisotropic electronic transport at half-filling factors in the second and third Landau levels. In the third Landau level at ν= 9 2 and 11 2 , the electronic transport is already strongly anisotropic in perpendicular magnetic field. This anisotropic phase is believed to be due to the formation of a unidirectional charge density wave. Applying an in-plane field overcomes the initial pinning and reorients the direction of this anisotropy. In the second Landau level, the even-denominator fractional quantum Hall state at ν= 5 2 and 7 2 is destroyed and transformed into an anisotropic phase by an in-plane magnetic field. This result provides additional evidence that the ground state at ν= 5 2 and 7 2 is spin-polarized.

We observe resonant tunneling into a voltage biased fractional quantum Hall effect (FQHE) edge, using atomically sharp tunnel barriers unique to cleaved-edge overgrown devices. The resonances demonstrate different tunnel couplings to the metallic lead and the FQHE edge. Weak coupling to the FQHE edge produces clear non-Fermi liquid behavior with a sixfold increase in resonance area under bias arising from the power law density of states at the FQHE edge. A simple device model uses the resonant tunneling formalism for chiral Luttinger liquids to successfully describe the data.

Magnetorotons in the dispersions of collective gap excitation modes of fractional quantum Hall liquids are measured in resonant inelastic light scattering experiments. Two deep magnetoroton minima are observed at $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2/5$, while a single deep minimum is resolved at $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/3$. The observations are the first evidence of multiple roton minima in gap excitations of the quantum liquids. The results support Chern-Simons and composite fermion calculations that predict multiple roton minima for states with $\ensuremath{\nu}g1/3$.

We have imaged the electronic compressibility of a two-dimensional electron gas in the integer quantum Hall regime. The compressibility images show quasi-insulating “incompressible” strips that separate region of electron liquid of near integer filling. The strips follow the contours of constant electron density that match exact occupancy of the Landau levels, and shift accordingly with magnetic field and electron density changes. The surface electrostatic potential distribution has an overall correlation with the compressibility patterns and the density contours marked by them and an abrupt step, at the boundaries, that provides a direct measure of the energy gap between the Landau levels.

We report the results of 3 terminal measurements on a one-dimensional (1D) wire. By utilizing cleaved-edge overgrowth, we were able to vary the coupling between a 1D conductor and an adjacent two-dimensional electron gas (2DEG) strip attached to the central part of the wire. We study the 1D-2DEG coupling by controlling the width of the 2DEG strip. Ballistic transport along the wire persists for interaction regions as long as 6 μm . Thus, with our shortest interaction region (narrowest strip) of 2 μm , the wire remains almost undisturbed by the presence of the nearby 2DEG allowing a true 3 terminal measurement on a clean wire. Our observations also explain the origin of the non-universal conductance quantization of such 1D wires.

Experimental data on the sequence of fractional quantum Hall effects (FQHE) states around Landau level filling factor ν =1/2 are well explained within the framework of the composite fermion model. Around other even-denominator fractions the situation is less clear, experimentally as well as theoretically. In this paper we report experimental studies at even-denominator fillings at ν=1/4, 3/4,5/2 , etc. We show that the composite fermion picture can be applied successfully to the fractions at ν =1/4 and 3/4. Their effective mass, when normalized to B 1/2 (where B is the magnetic field) and their effective g factor are very similar to the composite fermions at ν =1/2 and 3/2. Our ultra-low-temperature study at ν =5/2 shows that this fraction is a true FQHE state, showing vanishing R xx and a Hall resistance R xy precisely quantized to h /(5/2 e 2 ).

High spatial resolution imaging with terahertz pulses is implemented with a novel collection-mode near-field probe. The spatial resolution capabilities of the system are in the range of few micrometers. We demonstrate resolution of 7 /spl mu/m using 0.5-THz pulses and discuss performance of the collection-mode near-field probes and image properties.

The current-voltage power law exponent, $\ensuremath{\alpha}$, for electron tunneling into chiral Luttinger liquids at the fractional quantum Hall edge is found to exhibit a plateaulike structure at $\ensuremath{\alpha}$ close to 3 as the filling factor, $\ensuremath{\nu}$, is varied. The presence of a plateau near $\ensuremath{\alpha}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3$ strongly suggests a fundamental connection between $\ensuremath{\alpha}$ and the structure of the underlying quantum ground states associated with the robust incompressible $\ensuremath{\nu}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/3$ Hall fluid. However, the position in the inverse filling factor where the plateau occurs can vary between samples and appears shifted to values higher than expected from theory.

We studied the temperature dependence of nonlinear magnetoresistivity oscillations in high-mobility two-dimensional electron systems using a current-sweep method in a temperature range from 1.5 to 4.0 K. Consistent with previous studies we found that the primary source of the oscillation decay with increasing temperature is the modification of the quantum scattering rate entering the Dingle factor. These results provide further evidence that the high temperature decay of magnetoresistance oscillations appearing in the second order of the Dingle factor originates primarily from the electron–electron interaction effects.

We have measured magnetotransport at half-filled high Landau levels in a quantum well with two occupied electric subbands. We find resistivities that are {\em isotropic} in perpendicular magnetic field but become strongly {\em anisotropic} at $\nu$ = 9/2 and 11/2 on tilting the field. The anisotropy appears at an in-plane field, $B_{ip} \sim$ 2.5T, with the easy-current direction {\em parallel} to $B_{ip}$ but rotates by 90$^{\circ}$ at $B_{ip} \sim$ 10T and points now in the same direction as in single-subband samples. This complex behavior is in quantitative agreement with theoretical calculations based on a unidirectional charge density wave state model., Comment: 4 pages, 4 figures